Burnout physics/Exploits

Discussion in 'General Discussion' started by Nieubermesch, Jul 16, 2021.

  1. Nieubermesch

    Nieubermesch Registered

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    Recently on a exchange of ideas on another content release post by S397, I've come across a very interesting - at the very least - observation by @green serpent based on the tire physics of 3 specific cars by ISI or S397 (AC Cobra, Panoz Roadster and the Honda NSX).

    In his experiments he came across something that caught my eye in relation to sliding (longitudinal?) friction of the rear tires of those cars in comparison to the front grip, wich he found odd that the cars couldn't be made fixed on the front wheels while the rear wheels spun freely - presumably the realistic thing to happen.



    This is the video. The reason I think this isn't only a issue for someone who wishes to do burnouts, but also an issue with the behavior of the tires on the limit, in wich the occasionaly heard behavior of rotation on throtle that RF2 offers - that might or might not be realistic - can be related to...

    I tested it myself and the cars even with locked brakes on the front will still move forward with very low speed (tested with the new BMW).
    I mean, shouldn't the car remain fixed in place while the rear tires spin freely according with how much power is feeded? It makes me think about that if such a supposedly low friction can push the car forward like that, then when on static grip through a corner, it will help rotate and push the car in the direction the tires are pointing to with exaggerated accentuation and also be linked with exploits (possibly).

    Sorry about the mildly inchorent post, but I had to start something about this as I found it intriguing.

    Also should mention @mantasisg as he entered the brief and cut short conversation about it and left this video of real life burnout:



    The brief exchange can be seen under this post at pages 4 and 5:

    https://forum.studio-397.com/index.php?threads/bmw-m4-class-1-2021.69859/page-5

    Thanks.
     
  2. Korva7

    Korva7 Registered

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    Interesting indeed.
    Is there any other explanation than that the grip doesn't drop off like it should with high temperatures or high slip speeds?
    Does this happen with warm front tires also?
     
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  3. Nieubermesch

    Nieubermesch Registered

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    Well, I don'r really know the root cause of it, but the first assumption would be about grip, but I wonder if the front tires shouldn't also be leaving marks on the ground if they can be pushed like that and it's even weirder... I didn't test, but doubt warm tires will be different.
     
  4. Sim_Player

    Sim_Player Registered

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    I'm gonna give it a go on bmw m4.
    @Nieubermesch Always nice to find physics enthusiasts like you mate, wish more people talked about it than frikin clouds and trees.
    Anyways on topic, In case you haven't done it i suggest you few things
    1.Make brake bias all the way towards front
    2.As Korva7 said get front little bit warm.

    But in all honesty standing burnout shouldn't be too much problem with bmw or similar power cars(unless brake bias is very rearwards), extremely low surface grip.
     
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  5. Nieubermesch

    Nieubermesch Registered

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    Thanks mate! I like good looking trees as everyone else, but like good physics more ;).

    Yeah, I actually had to make the bias change anyway, as other way it wouldn't even spin at the rear, so that's done. Next time I'll do it with the tires warmer, but the grip of the track was also high I think.
     
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  6. Sim_Player

    Sim_Player Registered

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    oh okay then, will definitely give it a try.
     
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  7. Nieubermesch

    Nieubermesch Registered

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    Here is a video with the BMW M4 after having burned the rear tires off. It's moving very little, but it's rotating around and everything, when it shouldn't.
     
  8. green serpent

    green serpent Registered

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    In the spirit of Mythbusters, I decided to try and "replicate the results" by any means necessary.

    This was the formula.
    - Max rear tire pressure, min front tire pressure.
    -Totally nuked rear tires, fresh and warm front tires.
    - Doing the brakey slightly uphill.

    Even with all those advantages, it still wasn't happening. But then to my surprise a couple of times I was able to pull it off, perhaps the uphill gradient was at just the right angle.

    In this run the brakey didn't last for very long as my engine blew, but I kept the brakey going indefinitely on a few other runs. I included the tire wearing out process in the video for scientific purposes :D

    This run was actually a reasonably good example because you can see both a successful attempt at a brakey and then when I went for a second try, a brief failed attempt happened where the front tires push, despite still being uphill and the rear tires being totally worn out.



    Obviously, none of this is actually scientific, but IMO using man logic, anecdotal evidence and just general experience IRL and in sim, I feel that in general terms, rF2 tires when spinning have too much longitudinal and possibly also lateral grip. I think this is just another way of saying that rF2 tires don't have enough fall off, which I think is a fairly common sentiment at this point.

    Durning mid corner, when getting back onto the throttle, I feel like this issue manifests itself in two ways:

    1. Excessive amounts of understeer with partial throttle. Getting on throttle even very gently mid corner causes a lot of understeer pushing you wide. this is generally realistic as the rear tires push the front tires and also with an increase in speed the radius of the car must increase. However I feel, subjectively, that it is too excessive.

    2. Due to partial throttle causing a lot of understeer, this has lead to a habit of gunning the throttle harder mid corner to try and overcome the inherent understeer and push past it to get rotation. Here you find an actual benfit. There is almost never a sense of "spinning your wheels and getting nowhere" as the saying goes, no real sense of losing momentum with wheel spin. Instead the car seems to gain forward momentum and turn harder with slight oversteer than with positive steer. The car simply turns better with oversteer than normal steer (or at least no worse), which may be correct, but there is seemingly no trade off any loss of momentum. Cars in rF2 seem to display rapid changes in front trajectory mid corner, very pointy and darty front end behaviour, whereas IRL cars seem much more stable and smooth with a more liner arc through a corner, sim driving cornering can appear more polygonal in nature.

    Again all this is possibly not entirely fake, just a bit exaggerated. I am looking forward to the new tire being put on something with less downforce and really playing with the over the limit characteristics.
     
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  9. Korva7

    Korva7 Registered

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    I think differential setups plays into that. More locking under power -> understeer, unless the rear tires are slipping.
     
  10. green serpent

    green serpent Registered

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    Differential certainly plays into it. My comments were made in a very general sense. The cars in rF2 are so sensitive to set up changes you can pretty much get them to handle how you like.

    Also, as I stated in a different thread, I personally find that cars turn better with a more locked diff even under partial throttle as power gets tranmitted to the outer wheel, and seeing as the outer wheel has more grip than the inner wheel, it helps to rotate the car around the corner. As far as my driving style and my own experience, I find that the less power that the outer wheel gets, the less the car rotates on partial throttle (I'm sure the opposite would occur with different set ups/driving style etc)
     
    Last edited: Jul 17, 2021
  11. green serpent

    green serpent Registered

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    I tried the brakey experiment with one of my favorite recent mods - to me this car feels very natural to drive. Maybe it's down to coincidence, but it has also been the only car thus far to behave similar to what would be expected, actually quite close to that Cobra video from OP.

    No wearing out of rear tires or high rear tire pressures needed, just a slight change in front camber similar to what you'd see on a road car.



    Also, I just want to make it clear that if a car DOSN'T do this, I'm not saying straight of the bat that the tires are wrong. It is clear by the Cobra video that cars can certainly push the front while doing a brakey. Though, it is a bit odd that in certain cars, even with COMPLETELY worn out rear tires, highest possible rear camber and highest possible rear tire pressure, lowest possible front pressure, the rear fully spinning tires can still overcome the front locked tires. Unrealistic? Don't really know, but interesting. I do really like driving this BMW CS, but I do feel like it is a bit too plated at the rear.

     
    Last edited: Jul 17, 2021
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  12. Lazza

    Lazza Registered

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    I feel like you're overplaying the importance of the rears locking together and 'pushing' the car straight, vs the load transfer when you get off the brake (and then gently get on the throttle) if you're close to being front limited. If you are close to, or already exceeding, the front tyre grip, then you will alter the balance by moving the load away from the front tyres. It's the opposite of easing off the throttle when understeering mid corner to regain front grip.

    None of this is to say the majority of the rear tyres out there don't offer too much grip at higher slip angles, and given that contact patch shape can have a big bearing on this I wonder how much better the very recently released tyres (which I believe are [virtually] constructed differently to give better load reaction) may improve the shape of that lateral-longitudinal transition. The observations I read indicated they are a step in the right direction in this area.

    This seems to all sound about right - more than many other aspects of car setup the diff is one where the answer to the question "What will happen if I increase <this>?" is "Depends." The outer wheel is obviously outside the car centre of mass in relation to its current direction of travel, so a longitudinal force will help rotate the car (pushing a shopping cart with one hand won't make it go straight). An open diff spinning the inside tyre doesn't make the car lazy because the inside tyre is doing extra pushing (due to spinning), as much as the outside tyre isn't getting its fair share of the power. Too often setup guides offer a false dichotomy: more lock = understeer, less lock = oversteer. It's not that simple.
     
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  13. green serpent

    green serpent Registered

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    Yes, I can see how this might not be the case and that I've even contradicted myself to an extent. That being said, even if you remove that portion of my argument, I still feel that rear tires have to much push at times and don't spin as freely as they should, i.e they do not spin up easily enough, and then when they do they provide too much forward thust.

    One of my friends put it like this after he had been to one of those V8 supercar experiences. "If you give it too much throttle, at a certain point the rear tires just give up instantly and spin up, the grip disappears like the rear of the car is suddenly on ice, providing what feels like zero grip and complete loss of any forward thrust". This could be happening in sim and I just can't feel it, but I almost never (maybe on corner exit) feel like the rears are spinning up freely. It feels like they are drenched in glue.

    I wish there was a way to test this properly, this whole thing is just my feeling and hypothesis. I am all about actual objective evidence.

    One car that feels the most "correct" to me is the 911 Cup. With a good setup I love that car. Below video is an example of what I'm taking about regarding rear longitudinal grip. I have done this IRL a few times and in sim only with a small handful of mods like the Porsche, basically getting crossed up mid corner, keeping the power down and straightening up, but then the rear tires are still spinning for like 20 meters or something while the car is traveling in basically a straight line. The feeling is awesome! It is a bit hard to see, but the rear tires stop spinning at about the 23 second mark, well after the slide proper has ended.
     
    Last edited: Jul 17, 2021
  14. Comante

    Comante Registered

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    Unscientific + biased = (babyjesus × scientific method) x cry.
     
  15. Lazza

    Lazza Registered

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    There's some sort of tendency, maybe I'm only just noticing it and it's always been there, of everyone taking a side. I was only quoting and commenting on what you said about the effect of the diff lock on the handling. That's it. I made no comment on grip during wheelspin.
     
  16. green serpent

    green serpent Registered

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    I didn't really mean to imply that your comment was in anyway for or against what I was saying. I was simply stating that the first part of my explanation could indeed be incorrect, but that the other parts could still hold true. I can see that it seemed directed at you, but I was more just thinking out loud.
     
  17. green serpent

    green serpent Registered

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    As Pauline Hanson (Australian politician) would say, "Please explain!"
     
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  18. Rastas

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    rFactor 2 is great for drifting :)
     
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  19. Nieubermesch

    Nieubermesch Registered

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    In other words: Comante is a fanboy, don't mind him too much.
     
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  20. mantasisg

    mantasisg Registered

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    It is interesting test. I did this test to several of cars that I have been working on with physics. I was relatively confident they would succeed in this test, but they didn't. Perhaps it could be rF2 problem, but I doubt it a lot, it is probably about parameters of cars.

    However, there is one thing that I was thinking about. There is one variable that could be playing a part, perhaps a big part in this. IRL doing such stationary burnout the heat would not only affect the tire, but tarmac as well, tarmac would heat up locally and produce less friction from its part too. I remember reading some theory, that in case of extreme tire overheating, the tarmac with contact of a tire could have some instantaneous melting even for fast driving vehicle, this instantaneous thin melting film which further reduces tire rubber adhesion with a road. In case of these slow speed, static burnouts every single patch of tarmac will certainly overheat even more. There are some examples of burnouts that lasts long enough and without water applied (like people does in hot rod burnout shows) and digs holes in the tarmac.



    Quick google search says tarmac, depending on composition begins to melt at 50-170C. Which surely will be reached during standing burnout.

    Anyway, that is a little excuse I have thought out, because otherwise I'll have to tweak tires again of all my cars :D

    Now you'd be asking, why couldn't we just compensate this bit of tarmac overheating in tire overheating curve and or abrasive wear curve. Problem there would be that it would really need one new parameter, that would take account of how fast the vehicle is traveling to simulate in how much area tire heat would get dissipated to tarmac in single time unit, like tenth of a second, or have real road to have temperature memory for each of it's pieces, but I am afraid that would be a bit too much for rF2, especially as road surface temps are static to begin with, which is one of bigger lacks of rF2. However, I imagine people would find rF2 even more complicated if it would be even more realistic, driving at 10C road temperature for months not realizing it :D
     
    Last edited: Jul 17, 2021
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